Field of the Invention
The present invention relates generally to a device for testing ignition coils from automobile and other spark ignition engines, and more particularly to a small, handheld ignition coil tester which is powered by the battery of the engine's ignition system, and which provides a visual indication of proper high voltage pulse generation or, in the case of improper function of the ignition coil caused by a short in the ignition coil, provides an indication of the existence of the short in the ignition coil.
Spark ignition systems are used extensively in most small and intermediate size engines, such as those used in automotive applications. A spark ignition system is used to provide a spark in the cylinders of both two and four cycle engines to ignite the compressed air-fuel mixture to drive the pistons (or, in the case of a rotary engine, the rotors). The ignition system uses a battery and a generator to supply electrical power to the system, and a distributor having points or a breakerless impulse generation system, which together are used to supply ignition pulses to spark plugs located in each of the cylinders. The heart of the ignition system is the ignition coil, which is located between the power supply and the distributor. The ignition coil converts the low voltage of the power supply (the battery) to the high voltage pulses supplied by the distributor to the spark plugs.
The coil is essentially a transformer, with a primary winding and a secondary winding mounted on a common magnetic circuit with an air gap. One side of each of the primary and secondary windings are typically connected together, with the other sides of the primary and secondary windings being used for the low voltage input to the coil and the high voltage output from the coil, respectively. The primary to secondary turns ratio is typically 1:60 to 1:110, with the secondary typically having approximately 15,000 to 30,000 turns of 40- or 42-gage enameled wire. Typical primary resistance is 3.5 to 7 Ohms (a lower resistance is used in some coils which require an external resistor to operate properly), and typical primary inductance is approximately 8 to 20 mH.
For purposes of the discussion herein, the use of an ignition system having points will initially be assumed. The points are periodically opened and closed by rotation of the distributor shaft. The points are normally closed, supplying low voltage from the power supply to the primary winding in the coil. When a spark is to be produced, the points will be opened for a brief interval, with the sudden decay of flux in the primary winding of the coil producing a high voltage pulse from the secondary winding in the coil. The high voltage pulse is supplied by the distributor to the proper spark plug, producing a spark in the cylinder and initiating ignition of the compressed air-fuel mixture.
In a breakerless ignition system, an inductive or optical pickup is used instead of points. As the distributor rotates, electrical pulses are periodically generated by the pickup. The pulses are supplied to an amplifier, which generates an output signal which is used to drive the primary winding of the coil. Each time pulses from the pickup cause the amplifier to generate a low voltage pulse in the primary winding of the coil, a high voltage pulse is supplied from the secondary winding in the coil. The high voltage pulse is then supplied by the distributor to a spark plug, producing a spark in the cylinder and initiating ignition of the compressed air-fuel mixture.
It is thus apparent that the coil is the heart of the ignition system, and if it fails, the engine will be inoperative. It has thus been desirable to have some mechanism for testing a coil to determine whether or not it is good or defective. While an Ohmmeter may be used with limited success to check a coil to see whether it is defective, an Ohmmeter does not test a coil under actual operating conditions.
As might be expected, the art contains a number of references which to a lesser or greater extent address this problem. U.S. Pat. No. 2,249,157, to Morgan et al., U.S. Pat. No. 3,354,387, to Whaley et al., and U.S. Pat. No. 4,186,337, to Volk et al. are three such references. The Morgan et al. patent uses a large device with electrical circuitry to supply pulses to a coil and to analyze the output. Unfortunately, the Morgan et al. device does not provide a direct indication of the production of a spark by the secondary winding of the coil. Rather, the Morgan et al. device uses a complex (and bulky) electrical apparatus to determine whether the secondary coil has produced a proper output. A direct indication of this output is very much preferable, and the size and cost (due to its complexity) makes the Morgan et al. device interesting but not a complete solution to the problem.
The Whaley et al. patent is an improvement in that it is smaller, and it provides a more direct indication of proper coil operation. However, it uses an annular spark electrode in conjunction with another annular member somehow used as an electrode. The construction of these members is far from close to conducive to the generation of a spark, and their effectiveness may be somewhat less then optimal. In addition, the Whaley et al. device requires internal batteries, which increases the size and weight of the device.
The Volk et al. patent is further from the point, with a plurality of devices being required. The Volk et al. device also puts a spark indicator in series with the normal output pulse components, possibly inhibiting a spark from a normal coil due to the additional impedance of double spark gaps. The Volk et al. device also has far from optimal spark gap design, with spaced-apart cables being used as electrodes.
Other references of interest include U.S. Pat. No. 4,331,921, to Walker, which discloses a circuit for detecting an interruption of primary current caused by other ignition system problems; U.S. Pat. No. 4,401,948, to Miura et al., which discloses a system measuring the rise of secondary coil output voltage to determine stray capacitance in the ignition system; and U.S. Pat. No. 4,449,100, to Johnson et al., which discloses a device which evaluates the integral of secondary voltage over time. These devices are complex, for the most part expensive, and are not designed to determine whether a coil is functioning properly or not.
It is accordingly the primary objective of the present invention that it provide a small, inexpensive device which is capable of testing an ignition coil to determine whether or not it is functioning properly. The testing device must be easy to use, and it must provide direct connections to an automotive type coil so that it may be tested. The testing device should be operable using electrical power from an automotive battery rather than requiring internal batteries in the device, thus minimizing the size and weight of the testing device.
The testing device must include a signal generator to provide test signals to the primary winding in the coil, and visible means for indicating whether or not the coil being tested is in fact generating a high voltage output pulse from the secondary winding in response to the low voltage input signals being supplied to the primary winding. The visible indicating means must accurately simulate the spark gap in a spark plug, both to allow the operator of the coil tester to observe whether proper coil operation is occurring or not, and further to ensure that the device provides an electrically accurate analogy of the operation of a spark plug. It is a further objective that the coil tester provide an additional indication to confirm improper coil operation if the coil has an internal short.
It is an additional objective of the present invention that a tester be provided which is operable with all types of automotive coils, including those which require an external resistance to operate, and those that operate without such an external resistance. The improved tester of the present invention must also be of inexpensive construction to ensure it the broadest possible economic marketing advantage, and it must operate accurately and effectively over a long life achieved through durable construction. Finally, it is also an objective that all of the aforesaid advantages and objectives of the present invention be achieved without incurring any substantial relative disadvantage.